Ehleringer J., Mooney H. A., Gulmon S. L., Rundel P., Orientation and its consequences for Copiapoa (Cactaceae) in the Atacama Desert, Oecologia, 1980, 46(1):63-67, doi: 10.1007/BF00346967
The field study measures stem orientation and inclination in three historically recognised Copiapoa taxa, and their surface temperatures. A radiation balance model explains how northward orientation of the apex relates to warming of the growth and flowering zone.